1. Field of the Invention
The present invention relates to a developing apparatus used in image forming apparatus such as electrophotographic apparatus and electrostatic recording apparatus.
2. Related Background Art
There are a lot of conventionally known methods of electrophotography, for example as described in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910, and Japanese Patent Publication No. 43-24748. In ordinary methods, an electrostatic, latent image is formed on a photosensitive member by various means by the use of a photoelectric substance, the latent image is then developed by toner, a toner image thus obtained is transferred onto a transfer medium such as paper as occasion demands, and thereafter the image transferred is fixed by heating or solvent vapor or the like to obtain a copy.
There are also a variety of developing methods for visualizing the electric, latent image by use of toner. For example, many developing methods are known, including the magnetic brush developing method as described in U.S. Pat. No. 2,874,063, the powder cloud method and fur brush developing method as described in U.S. Pat. No. 2,221,776, and the liquid developing method.
Among these developing methods, the magnetic brush developing method using a two-component developer mainly containing toner and carrier is an excellent method capable of obtaining high-quality images relatively stably and is popularly used in practice. However, the magnetic brush developing method using the two-component developer has drawbacks of deterioration of carrier and variation in mixture ratios of toner and carrier, because it uses the two-component developer. In addition, it requires an agitating member, a toner concentration sensor, and the like to be set in the developing unit in order to overcome the drawbacks. The problem was, therefore, that the developing unit itself became large.
For avoiding this problem, a variety of developing methods using a one-component developer containing only toner have been proposed. For example, U.S. Pat. No. 3,909,258 proposed a method for developing the latent image by use of magnetic toner having the electrically conductive property. This is a method for developing the electrostatic, latent image by supporting the magnetic toner on a cylindrical, conductive sleeve (developing sleeve) having a magnet inside and making the toner contact the latent image. On this occasion, toner particles form conductive paths between the surface of photosensitive member and the surface of developing sleeve in the developing section, the charge is guided via these conductive paths from the developing sleeve to the toner particles, and the toner particles adhere to image portions by Coulomb force with respect to the image portions of the electrostatic, latent image, thereby developing the image. This developing method using the conductive, magnetic toner is an excellent method avoiding the problem in the conventional two-component developing method, but, on the other hand, it has a drawback that, because the toner is electrically conductive, it is difficult to electrostatically transfer the developed toner image from the photosensitive member to a final support member such as plain paper.
For solving this problem, a developing method using high-resistance toner to enable the electrostatic transfer was proposed; for example, Japanese Patent Application Laid-open No. 52-94140 describes a developing method utilizing dielectric polarization of toner particles. This method, however, has such drawbacks that the developing speed is essentially slow, the density of developed image is not sufficient, and so on, and it was not practically applicable. Other known methods using the magnetic toner of high resistance are those for frictionally electrifying the toner particles by friction between toner particles, friction between toner particles and developing sleeve, etc. and making them contact the photosensitive member, thereby developing the electrostatic, latent image. It is, however, pointed out that these methods have such drawbacks that the amount of contact is small between the toner particles and a frictional member, so as to result in insufficient frictional electricity (triboelectricity) and that when the Coulomb force is strong between the charged toner particles and the developing sleeve, the toner particles are likely to aggregate on the developing sleeve, and therefore, they have many difficulties in practical use.
Against it, Japanese Patent Application Laid-open No. 54-43038 proposes a novel developing method eliminating the above drawbacks. This is a method for making an elastic blade of rubber or metal contact the developing sleeve, thereby frictionally electrifying the toner on the developing sleeve and forming a thin layer of the toner thereon, and then bringing it to the very vicinity of the electrostatic, latent image under action of magnetic field to keep the toner layer opposed thereto without contact, thereby developing the latent image. This method increases the chances of contact between the magnetic toner and the developing sleeve by making the very thin coating of magnetic toner, thereby enabling to give the toner an amount of triboelectric charge necessary for development. However, since the magnetic toner includes a magnetic material such as magnetite, this method has problems that it is difficult to apply it to color toner and that it is very difficult to decrease power consumption, because the fixing temperature must be set rather high.
On the other hand, a non-magnetic one-component developing method not using the magnetic toner is proposed and used in practice, as described in Japanese Patent Application Laid-open No. 58-116559. Since this method can be applied to formation of color image and can realize a inexpensive and compact developing apparatus, it is frequently used in developing units of printer or the like.
FIG. 8 shows a developing apparatus using the conventional, non-magnetic one-component developing method. This developing apparatus has a developing sleeve 11 in an opening portion of developing container 13 for accommodating the non-magnetic toner of one-component developer. This developing sleeve 11 is opposed with a predetermined clearance to the photosensitive drum (not illustrated) and is driven to rotate in the direction of the arrow. A toner supply roller 14 is set in contact with the developing sleeve 11 inside the container 13. The supply roller 14 is constructed in such manner that an elastic member 14b of urethane foam or the like covers the outside periphery of core 14a of SUS or the like. The supply roller 14 frictionally rotates in the direction of the arrow relative to the developing sleeve 11, so as to supply the non-magnetic toner in the container 13 to the surface of the developing sleeve 11 to form a coating of toner thereon, and it also scrapes off the residual toner not having contributed to development from the surface of developing sleeve 11. A regulating blade 12 is in contact with and on an almost top part of developing sleeve 11. This regulating blade 12 is constructed in such structure that an elastic member 12b of urethane rubber or the like is bonded to a surface of elastic support member 12a of phosphor bronze or the like on the opposed side to the developing sleeve 11 and the regulating blade 12 functions to form a thin layer of toner by regulating the non-magnetic toner supplied to the surface of developing sleeve 11 and to give the charge to the toner.
By employing the above construction, the non-magnetic one-component developing apparatus well forms the thin layer of non-magnetic toner on the developing sleeve 11, so that it can well develop the electrostatic, latent image on the photosensitive drum.
However, when the coating of toner was formed on the developing sleeve 11 by frictional rotation of the toner supply roller 14 in contact with the developing sleeve 11, non-charged toner was often supplied onto the developing sleeve 11, depending upon the placement location and rotation direction of the toner supply roller 14, and upon the placement location of the regulating blade 12.
When an excessive amount of coating of non-charged toner was supplied onto the developing sleeve 11, the regulating section by the regulating blade 12 sometimes failed to give the sufficient charge to the non-charged toner, so that deviation of in-layer charge amount distribution could appear in the toner layer formed on the developing sleeve by regulation, which sometimes caused negative effects such as fog and nonuniformity on the developed image. In addition, the method had a drawback that the load on the toner was very heavy in the contact portion between the toner supply roller 14 and the developing sleeve 11, so that the toner was thus likely to be deteriorated thereby.
According to studies by the inventors, the above toner supplying means sometimes failed to perfectly remove the toner remaining on the developing sleeve 11, so as to leave some of it on the developing sleeve. As a result, the residual toner passed many times between the regulating blade 12 and the developing sleeve 11 and between the toner supply roller 14 and the developing sleeve 11. It was thus found that frictional heat upon passage accumulated in the residual toner and quickly deteriorated the toner on the developing sleeve 11. This deteriorated toner could fuse to the nip portion of regulating blade 12, which would be a cause to induce failure in toner coating, e.g., dripping thereof.
Since the residual toner and new toner was mixed on the developing sleeve 11, development hysteresis such as ghosting often occurred. Further, continuation of use gradually increased the charge amount of residual toner, which impeded supply of new toner and degraded the development property because of decrease in coating amount of toner in some cases. In addition, after the toner supply roller 14 was rubbed on the developing sleeve 11 for a long period, the roller 14 itself was worn or damaged, thereby making the supply and stripping of toner insufficient.
Therefore, the one-component developing method using the non-magnetic toner as described above is mainly used in cartridge type developing apparatuses which are poor in endurance stability of toner and developing apparatus and which are replaced entirely when the toner is used up, and it is not often employed in the developing apparatuses of the type to be replenished with developer, such as copiers.
An object of the present invention is to provide a developing apparatus capable of supplying only sufficiently charged toner to a developer carrying member.
Another object of the present invention is to provide a developing apparatus capable of coating a developer carrying member with toner without applying pressure to the toner.
Still another object of the present invention is to provide a developing apparatus capable of preventing deterioration of toner.
Still another object of the present invention is to provide a developing apparatus comprising:
a developer carrying member for developing an electrostatic image on an image carrying member with toner;
clouding means for forming a toner cloud near said developer carrying member; and
coating means for coating said developer carrying member with the toner in the toner cloud by force of electric field.
Further objects of the present invention will become apparent by the descriptive matter which follows.